23 December 2005

Einstein's famous equation that equates energy to matter, E=mc2, is one of the fundamental building blocks of modern physics. In yesterday's Nature, Rainville et al.†, reported the results of their tests of the accuracy of this equation.

A direct way to test the validity of Einstein's equation is to independently measure the mass lost (Δm) during a particular process and the actual energy (E) released and then compare the latter with the energy equivalent of the former. By dividing both sides of E=Δmc2 by E, we get 1=Δmc2/E. The rearrangement of the latter gives 1-Δmc2/E=0. In other words, if the energy released and the mass lost during a process are equal as predicted by Einstein's equation, then Δmc2/E=1 and therefore, 1-Δmc2/E=0.

When an atomic nucleus captures a neutron (n) it moves up to an excited state (marked by an asterisk below). Upon returning to the lower energy ground state, the nucleus emits a gamma-ray photon. Using a sulfur (S) atom as an example, this can be represented as follows.

32S + 1n --> 33S* --> 33S + gamma-ray

Rainville et al., measured the mass difference (Δm) between isotopes of sulfur (and silicon) and unbound neutrons and the final isotopes incorporating captured neutrons. The multiplication of Δm with c2 (c=speed of light) gave the energy equivalent of mass lost during the above process. They also measured the energy (E) of the gamma-rays released. Their combined results obtained with sulfur and silicon gave the value of

1-Δmc2/E=-0.00000014

Because of various errors involved in measurements, no experimental result for the above equation would ever be exactly 0. Rainville et al., state that their result is "55 times more accurate than the previous best direct test of E=mc2".